Seasonal changes in phytoplankton composition in an extensive fish culture area in Bolinao, Pangasinan, northern Philippines

Abstract

The proliferation of fish cages and pens in Bolinao, Pangasinan, northern Philippines seemed to have reached its limit in 2002 when a massive fish kill coinciding with a bloom of Prorocentrum minimum occurred. Since then, various measures have been taken by the local government to decrease the number of fish cages to allow recovery of the waters of Bolinao. Samples collected in April, June and November 2004 representing summer, southwest and northeast monsoons, respectively, from five stations were analyzed for phytoplankton composition. Diatoms generally dominated the phytoplankton and peaked during summer which can be attributed to a very high concentration of Thalassiosira sp. constituting 92% of the phytoplankton in that season. The dinoflagellates constituted a small portion of the phytoplankton composition in all seasons. The flagellates represented a small fraction of the phytoplankton and followed the trend of the dinoflagellates. Cyanobacteria thrived during the wet seasons, almost co-existing with the diatoms. The results seem to suggest that the management of the fish cages or the aquaculture ponds might have resulted to changes in the present (2004) phytoplankton composition compared to previous years (1995-2002)